If you are monotheistic, then you believe in one god. Christianity, Judaism, and Islam are monotheistic religions.
These days the monotheistic religions — ones that believe in one god — are the most popular. But other religions, like Buddhism and Hinduism, are polytheistic — meaning they believe in many gods. Usually in a monotheistic religion, God (with a capital G) is an all-powerful being who created the universe and knows everything. Monotheistic religions tend to look at other types of believers as pagans, heathens, and heretics.
